1. Field of the Invention
The present invention relates to a method of enabling a light beam emitted from, for example, an optical head to access a track on a card-shaped recording medium.
2. Description of the Related Art
An optical card or a card-shaped recording medium has a memory capacity which is about a few thousands through ten thousand times larger than the memory capacity of a magnetic card. Since the optical card has a memory capacity of 1 to 2 megabytes, it is applicable to various uses: deposit passbooks, portable maps, prepaid cards, etc.
Various types of optical cards have been proposed. The applicant of the present application proposed, in Published Unexamined Japanese Patent Application No. 63-37876, an optical card as shown in FIG. 1. An optical card 11, shown in FIG. 1, includes an optical recording section 13 having a plurality of parallel tracks 12. ID areas 14A and 14B are provided at both end portions of the optical recording section 13. The ID areas 14A and 14B store address data corresponding to the respective tracks 12, thus enabling data to be read in either direction. A data area 15 is provided between the ID areas 14A and 14B. The track address data corresponding to the tracks of the optical card 11 is recognized by reading the ID area 14B when the card 11 is moved relative to an optical head from the left (in FIG. 1) to the right along the tracks 12 or by reading the ID area 14A when the card 11 is moved from the right to the left reactive to the optical head.
FIG. 2 shows the structure of a main part of an optical card apparatus using the optical card 11. The card 11 is moved in X-direction and an optical head 21 is moved in Y-direction perpendicular to X-direction, thereby recording/reproducing data. The optical head 21 includes a laser diode 21A. A light beam emitted from the laser diode 21A is projected onto the track on the optical card 11 through a collimator lens 21B, a prism 21C and an objective lens 21D. The beam is reflected from the track of the card 11 and received by a light detector 21E through the objective lens 21D and prism 21C. An output from the light detector 21E is fed to a signal processing circuit (not shown). The signal processing circuit detects a read-out signal, a focus error signal and a tracking error signal. The objective lens 21D is driven in the focusing direction and tracking direction (Y-direction) on the basis of the focus error signal and the tracking error signal. Namely the beam is controlled so as to be kept in the focused state in relation to the track on the card 11. The optical head 21 is provided with a linear scale 22 extending in Y-direction. The position of the optical head 21 in Y-direction is detected by the scale 22 and the scale detector 23.
In the optical card apparatus, the light beam to be projected onto the optical card 11 is shifted between different tracks in the method: 1) the entire body of the head 21 is moved in Y-direction and the head 21 is positioned by the linear scale 22 and scale detector 23 ("head seek" or "rough seek"), or 2) the objective lens 21D is shifted in a track-by-track manner ("track jump" or "fine seek").
The first method is used for quick shift between tracks which are situated at a relatively long distance. According to the first method, there is a low possibility that the light beam can be precisely positioned at a target track after the shift. The reason is that the precision of the linear scale 22 is not satisfactory and the position of the center of the objective lens may be displaced.
The second method is used for precise shift between tracks which are situated at a relatively short distance. According to the second method, the beam is surely jumped track by track, and therefore the beam can be positioned at a target track with high possibility, compared to the first method. However, the second "track-by-track" method requires a longer time for the shift between the tracks situated at a long distance, than the first method.
In the actual accessing process, the first and second methods, in general, are combined, as illustrated in FIG. 3. In starting data read/write, a difference D between the track address of the target track and the track address of the present track (at which the head is presently situated) is found. It is determined if the absolute value of D is less than a predetermined value r (normally 4 to 10). If the absolute value D is less than r, track jump (fine seek) is carried out for a distance corresponding to D. If the absolute value D is r or more, head seek (rough seek) is carried out for a distance corresponding to D, and the head is moved to the vicinity of the target track. Thereafter, the optical card 11 is moved in X-direction and the track address on the ID area 14A or 14B is read. If the track address which has been read coincides with the target track, the data read/write is executed. If they differ, the above operation is repeated. Normally, the target track can be accessed by the track jump access, unless the card has some defects.
The above accessing method is generally employed in the field of optical discs, etc.
In the above conventional accessing method, however, a seek error may occur and the target track cannot be accessed, if a region near the target track has some defect.
In the case of an optical card, unlike an optical disc, a recording/reproducing beam is moved relatively along the tracks in a linear and reciprocal manner. Thus, the speed of movement of the optical card, relative to the recording/reproducing beam, is not constant. In general, the speed of movement of the inner area of the card (including ID areas) is substantially constant, and that of the outer areas (outside the ID areas) is decreased because of temporary stop and start of movement of the card. On the other hand, if there is a defect on the optical card, a tracking servo is disturbed and a tracking error may occur. The possibility of tracking error becomes higher, as the speed of movement of the card decreases, because the time period at which the head is passing over the defective point increases. In the case where the speed of movement of the card is low, even a small defect on the card may lead to tracking error. In fact, tracking errors due to defects on the card are caused, in many cases, by the defects on the areas outside the ID areas.
The occurrence of the seek error in a conventional access method will now be described with reference to FIG. 4. In FIG. 4, reference numeral 16 denotes a beam spot, 17 a guide track, and 12a a target track.
In the initial state, the beam spot 16 is situated at a location relatively far away from the target track 2a. The beam spot 16 is brought to the vicinity of the target track 12a by the head seek, as indicated by arrow 1. Then, with the movement of the optical card 11 in a first direction along the tracks, the spot 16 is moved, as indicated by arrow 2, and the ID area 14B is read. In this case, since the spot 16 is close to the target track 12a, it is shifted to the track 12a by track jump, as indicated by arrow 3. Thus, the beam spot 16 is positioned at the target track 12a. Then, with the movement of the card 11 in a second direction along the tracks, the spot 16 is moved, as indicated by arrow 4. If there is no defects on the card, the track address is confirmed by reading the ID area 14A, and subsequently, data read/write is carried out on the target track 12a.
If a defect (indicated by x) is present on the optical card 11, the tracking error occurs and the beam spot 16 is displaced to a non-target track, as shown by arrow 5. Consequently, the beam spot 16 scans the non-target track, as indicated by arrow 6. In this case, the beam spot 16 reads the ID area 14A to confirm the track address. If the amount of the tracking error indicated by arrow 5 is large, the difference D between the target track 12a and non-target track increases; thus, the head seek is carried out once again. Namely, the beam spot 16 is moved to the target track or a track near the target track, as indicated by arrow 7. Then, the beam spot 16 scans this track to which the spot 16 has been moved as indicated by arrow 8, with the movement of the optical card 11 in the first direction. and also reads the ID area 14B to confirm the track address. In this case, the track which has been read is not the target track 12a, subsequently track jump is performed, as indicated by arrow 9, whereby the beam spot 16 is brought to the target track 12a.
The above operation is repeated until the beam spot 16 is positioned at the target track 12a. More specifically, this operation is repeated until the amount of tracking error indicated by arrow 5 is so small that the beam spot 16 can be shifted to the target track 12a by "track jump" over the distance indicated by arrow 7, or until the spot 16 is incidentally shifted to the target track 12a by single "head seek" over the distance indicated by arrow 7. In this way, the accessing process is repeated several times until the beam spot 16 is positioned to the target track 12a; however, during the accessing process, a further tracking error may occur owing to a defect (indicated by x) on the optical card 11 so that the access to the target track cannot be attained.